The present invention relates generally to a polarization converter, and more particularly to a test apparatus incorporating the same.
Military weaponry, such as missiles, oftentimes employ one or more transmitters or receivers to facilitate high frequency communications. These communications typically may be used in guiding the missile, tracking the missile, etc. In such instances, the transmitter or receiver typically includes one or more antennas for transmitting/receiving signals used in the guidance or tracking of the missile. The transmitter, receiver and antennas may be located in the nose of the missile and may communicate with guidance or tracking equipment located on the ground. Alternatively, the transmitter, receiver and antennas may communicate to engage a target, etc.
The antennas may be linear or circular polarized, for example. However, circular polarized antennas are oftentimes preferred. A circular polarized communication signal and antenna tend to be less susceptible to interference from reflections due to ground clutter, etc.
In view of the critical nature of such missile communications for guiding, tracking, etc., it is very important that the missile communications operate as intended. Consequently, xe2x80x9choodsxe2x80x9d have been used extensively to verify the performance of the antennas, transmitters, receivers, cables, etc. that have been installed in the missile. For example, a hood is placed around the missile body and the power radiated from the antenna within the missile is detected by the hood and compared to a standard to determine if the hardware is functioning properly.
A problem arises, however, when the antenna within the missile is circularly polarized yet has a less than perfect axial ratio. An imperfect axial ratio can create large variations in the power sensed by the hood. Such variations are further increased by the fact that the hood is typically working in the near field of the antenna being tested. Variations in the radiated power levels sensed by the hood could often be 10 decibels (db) or more. These variations could be sufficient to mask a faulty antenna, transmitter and/or cable within the missile.
Consequently, it was difficult in the past to determine with certainty if an alarming variation in the radiated power sensed by the hood was due simply to a less than perfect axial ratio, or instead faulty hardware within the missile. Large variations had to be resolved by disassembling the missile and testing the antenna, transmitter, cables, etc. individually. This was quite time consuming and expensive as it required a significant number of skilled man-hours. Even if the antenna, transmitter, cables, etc. were tested individually prior to assembly within the missile, there still would be uncertainty as to whether there was damage or failure during installation in the missile.
In view of the aforementioned shortcomings associated with testing the missiles, there is a strong need for an apparatus which overcomes the problems associated with large axial ratios. More specifically, there is a strong need for a hood apparatus which can verify with certainty if the installed antenna, transmitter, cables, etc. are functioning properly even in the event of a large axial ratio. There is a strong need for such a hood apparatus which provides such verification so as to eliminate the need to disassemble the missile in order to test the components individually.
In accordance with the invention, a hood is provided for verifying the performance of a missile. The hood includes one or more sense antennas having the same polarization as the antennas within the missile to be tested. The hood further includes one or more elliptic-to-circular polarization converters designed to reduce the axial ratio of the signal received from missile. The elliptic-to-circular polarization converters convert the signal from the missile to a substantially pure circular polarized signal which is then sensed by the sense antennas. This eliminates the uncertainty associated with conventional hoods as to whether variations in the sensed signal are due to imperfect axial ratio or a failure of one or more components within the missile.
According to one particular aspect of the invention, an elliptic-to-circular polarization converter is provided for converting an elliptically polarized signal traveling along a propagation path into a circularly polarized signal. The converter includes a grating including electrically conductive members arranged to separate a linear component associated with the elliptically polarized signal from a circular component associated with the elliptically polarized signal, and to allow the circular component to travel through the grating along the propagation path while blocking travel of the linear component along the propagation path.
According to another aspect of the invention, a test apparatus is provided for evaluating a signal source with potentially a large axial ratio. The test apparatus includes an elliptic-to-circular polarization converter for converting an elliptically polarized signal traveling along a propagation path, from the signal source, into a circularly polarized signal, the elliptic-to-circular polarization converter including a grating including electrically conductive members arranged to separate a linear component associated with the elliptically polarized signal from a circular component associated with the elliptically polarized signal, and to allow the circular component to travel through the grating along the propagation path while blocking travel of the linear component along the propagation path. The test apparatus further includes a sense antenna for receiving at least part of the circular component allowed to travel through the grating along the propagation path.
In accordance with yet another aspect of the invention, a missile hood for evaluating a signal source within a missile is provided. The missile hood includes an elliptic-to-circular polarization converter for converting an elliptically polarized signal traveling along a propagation path, from the signal source, into a circularly polarized signal. The elliptic-to-circular polarization converter includes a grating including electrically conductive members arranged to separate a linear component associated with the elliptically polarized signal from a circular component associated with the elliptically polarized signal, and to allow the circular component to travel through the grating along the propagation path while blocking travel of the linear component along the propagation path. The missile hood further includes a sense antenna for receiving at least part of the circular component allowed to travel through the grating along the propagation path; and an annular housing assembly fittable over a body of the missile for holding the grating and the sense antenna in fixed series relation along the propagation path relative to the signal source.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.